1. Field of the Invention
This invention relates to a focal length varying device for automatically varying the focal length of the photo taking optical system of a camera having a variable focal length and more particularly to a focal length varying device for a camera which reliably provides an adequate picture even when photographing at a long focal length.
2. Description of the Related Art
Heretofore various contrivances have been proposed for a camera of the kind having a non-interchangeable lens of variable magnification, or a variable focal length. Some of these proposals have already been put in practice. Camera users are favorably receiving such cameras as they give a broader photographable range than the conventional non-interchangeable lens type camera having only a short focal length.
However, the known proposals for the camera of the above stated kind and the cameras already marketed embodying these proposals have the following drawbacks:
(i) Variations in focal length alter the depth of field and thus tend to blur the image of an object to be photographed. However, there is provided no solution of this problem.
In the compact cameras of the kind permitting selection between photographing at a long focal length (hereinafter referred to as TELE mode) and photographing at a short focal length (hereinafter referred to as WIDE mode), the depth of field becomes significantly shorter in the TELE mode than in the WIDE mode. For example, when a three-dimensional object is photographed in the TELE mode, the images of front and rear parts of the object are greatly blurred.
(ii) The camera has its taking lens designed to be drawn forward in the TELE mode. The focus adjustable range for a near object distance, therefore becomes narrow in the TELE mode. In the TELE mode, an object located even slightly nearer than a focused distance results in a blurred picture.
(In the above stated camera, focus adjustment for the near distance range is arranged to be effected by drawing the photo taking lens forward. Hence, in order to avoid an excessively narrow focusable range for the near distance range, the taking lens must be arranged to have an ample room for the forward movement of the lens. Whereas, in the case of the TELE mode, since the lens has already been drawn forward, the lens cannot be drawn further forward to a sufficient extent from that position unless the camera is allowed to have a greatly increased thickness. However, an increase in thickness is undesirable as it makes the camera larger. Accordingly, cameras have been kept in a compact size at the expense of the focus adjustable range for the near distance range.)
(iii) The exposure interlocking range of operation of the prior art camera is much narrower in the TELE mode than in the WIDE mode. Hence, in the TELE mode, it is difficult to obtain a sharp picture and a photographing operation often becomes impossible. The reasons for these are as follows:
With the lens aperture assumed to be unvarying, the F number naturally increases with the focal length. This narrows the exposure interlocked range of operation. As a result, the exposure interlocking range of operation of the camera is narrower in the TELE mode than in the WIDE mode. In addition, shaking of the camera due to hand vibrations becomes more conspicuous as the focal length increases (or as the magnifying power increases). Therefore, the threshold shutter speed must be set at a faster speed for the TELE mode than for the WIDE mode.
The exposure interlocking range of operation of the camera in the TELE mode is thus much narrower than in the WIDE mode because of both the influence of the F number and that of the above stated threshold shutter speed. The TELE mode, therefore, tends to result in a faulty photographing operation.